The field of the invention relates to the removal of mercury from a liquid such as water or hydrocarbon condensate.
Hydrocarbons, both gas and condensate, produced from certain natural gas fields have been found to contain significant amounts of mercury. The gas and condensate from one field, for example, are found to have mercury contents of about 250 and 200ppb, respectively. The presence of mercury in the gas and condensate causes both processing and environmental concerns.
Water co-produced from gas and oil wells also may contain significant amounts of mercury. For example, concentrations of 70-150 ppb of mercury have been observed in water produced from gas wells in certain natural gas fields. In addition, mercury-contaminated water is produced in various manufacturing processes. The discharge streams from chlor-alkali plants have sometimes been found to contain unacceptable mercury levels. It is environmentally unacceptable to discharge such mercury-containing effluent to rivers or oceans.
Various processes have been developed for removing mercury from liquids. U.S. Pat. Nos. 2,860,952, 3,674,428, 3,749,761, 3,790,370, 3,847,598, 4,053,401, 4,147,626 and 4,474,896 disclose a number of methods for reducing the mercury content from aqueous solutions. Most of these methods involve the addition of certain chemicals to the solution to precipitate the mercury compounds or the use of adsorbents. U.S. Pat. No. 3,847,598 discloses a process including passing a stream of inert gas through an aqueous solution in the presence of a reducing agent and subsequently recovering mercury vapor from the inert gas. The process is used for treating depleted brine used in the manufacture of chlorine and caustic soda.
Mercury removal processes for treating water which involve the use of sulfides can be enhanced by the addition of polysulfides. However, such processes are relatively costly and require close control of conditions. The treated stream has a reduced mercury content but is greatly enriched with sulfides and COD, therefore requiring further treatment. Furthermore, it is difficult to remove the fine particles of HgS from the system.
Mercury in water can also be removed by passing it over active adsorbents such as sulfur/carbon, Ag/A1.sub.2 0.sub.3, Ag/C, and CuS/A1.sub.2 0.sub.3. Water produced at natural gas wells may contain contaminants other than mercury which can foul the adsorbents.
A technique for removing mercury from condensate has involved washing the condensate with a dilute alkali solution of Na.sub.2 S.sub.x. The consumption rate of the Na.sub.2 S.sub.x is high due to the fact that many compounds in the condensate compete with mercury for reaction with the Na.sub.2 S.sub.x. In addition, the by-product of this process causes disposal problems.